These storage tanks offer a great, compact way of storing fuel on your roof rack, outside of the car. The main drawback is that they're freaking expensive- $90 for one. Also read the reviews on the site. The one common complaint is the vent valve is in a vulnerable position. I never had an issue, but it was always noticed.

You can just barely make out two of them strapped between the rack bars on my roof in this image (click for full sized):


J